1. Field of the Invention
The apparatus of the present invention relates to improved receiver/timing circuitry for utilization in single frequency, time-shared, continuous wave, phase-comparison positioning systems.
2. Description of the Prior Art
Single frequency, time-shared, continuous wave, phase-comparison positioning systems are utilized extensively today primarily as a means for determining the location of a vessel at sea. The location of the vessel is extremely important when, for example, it is conducting seismic operations. The seismic records that are gathered at sea must be correlated with the location at which they were gathered so that later drilling operations may be commenced in the proper location.
A typical single frequency, time-shared, continuous wave, phase-comparison positioning system is shown in FIG. 1. It comprises a master station and two slave stations which transmit continuous wave signals for fixed intervals, in fixed sequence, and at a fixed repetition rate. The repetition rate is usually chosen as one second, with the master station transmitting interval the first 400 milliseconds and with each slave station transmission interval being 300 milliseconds. During the first 100 milliseconds of the master station transmitting interval, the master station transmits a trigger signal which is utilized by the receivers in the slave stations and the vessel at sea to lock their timing circuitry to the master. During the last 900 milliseconds of a one second period, the master and slave stations transmit pattern frequency signals. The pattern digitizer circuitry utilizes the pattern frequency signals to determine the location of the vessel.
The frequency of transmission that is used by the master and two slave stations will, of course, have a given wavelength. The receiver on the vessel at sea compares the phase difference between the master station pattern frequency transmission and the first slave station pattern frequency transmission and the phase difference between the master station pattern frequency transmission and the second slave station pattern frequency transmission to accurately determine its position at sea. This phase difference is calculated by the pattern digitizer which outputs a visual display of the location of the vessel.
Single frequency, time-shared, continuous wave, phase-comparison positioning systems of the prior art have depended completely upon the reception of a proper trigger signal from the master station to initiate circuitry for the phase-comparison described above. In ideal weather conditions, the positioning systems of the prior art have worked relatively well, but their performance has been susceptible to being adversely affected.
Adverse weather conditions is an example of an instance in which the performance of the positioning systems of the prior art may be adversely affected. During an electrical storm, for example, electronic signals will be generated by lightning, and the electrical signals generated by this lightning may be detected by the receiver on the vessel as the trigger signal from the master. A trigger so detected is a false trigger, but systems of the prior art could not discern false triggers from good triggers. Consequently, the false trigger reinitialized timing circuitry and phase-comparison would be made between pattern frequency signals that should not be compared. The pattern digitizer electronics on the vessel would change rapidly when the erroneous comparison was made, and the correct position of the vessel at sea would be lost. Consequently, further data could not be gathered since the data would have no location with which to be correlated. The vessel would have to return to a location (usually the shoreline) whose coordinates are known to obtain accurate location information to be entered into the pattern digitizer. After returning and obtaining its correct coordinate positions, it would return to the location where operations were being previously conducted and then resume operations. If the vessel were, for example, operating at a distance of 100 miles offshore, it is apparent that a considerable period of time is lost when the vessel must travel 200 miles before meaningful operations can begin again.